1. Field of the Invention
This invention relates to a latent catalyst extremely excellent in moisture resistance reliability, curability and storage stability at ordinary temperature, a thermosetting resin composition comprising the same, an epoxy resin molding material for encapsulating a semiconductor responding to insertion mounting and surface mounting which molding material comprises the above latent catalyst and is excellent in moisture resistance reliability, storage stability at ordinary temperature and moldability, and a semiconductor device in which the molding material is used.
2. Related Art Statement
Thermosetting resins have been required to have a storage stability at ordinary temperature for the purpose of simplifying the handling thereof, and various latent catalysts have been developed by now. The latent catalyst means a catalyst having excellent curability and excellent storage stability at ordinary temperature for exhibiting its catalytic activity at an appropriate temperature or higher. For example, JP-A-8-295,721 discloses that a tetra-substituted phosphonium tetra-substituted borate in which the substituents on boron have a specific structure has both excellent curability and excellent storage stability at ordinary temperature. The tetra-substituted phosphonium tetra-substituted borate exhibits its catalytic activity upon dissociation of the ionic bond between the anion portion and the cation portion, so that the temperature at which the catalytic activity is exhibited can be varied by changing the temperature at which the bond is dissociated. When the substituents on boron have a specific structure, the ionic bond comes to have an appropriate bond-strength and exhibits no activity at ordinary temperature but is dissociated at the curing temperature to rapidly exhibit activity. Therefore, excellent curability and excellent storage stability, namely latency, is developed. The field in which such a latent catalyst takes an important part includes an epoxy resin molding material for encapsulating a semiconductor such as IC, LSI or the like. As the catalyst which has heretofore been used, there are imidazoles, diazabicycloalkenes, triarylphosphines and the like; however, these catalysts act even at relatively low temperatures, so that molding materials in which these catalysts are used are inferior in storage stability at ordinary temperature. Therefore, when these molding materials are stored at ordinary temperature, such troubles arise that incomplete filling results from the lowering of flow property during molding, gold wire in IC chips is broken to cause conduction failure, and the like. Therefore, it has been essential to refrigerate the epoxy resin molding material for encapsulating a semiconductor (referred to hereinafter as the encapsulating material in some cases) when it is transported and stored. Moreover, recently, the performance required for IC as to the moisture resistance reliability has become strict more and more. For example, JP-A-7-242,683 mentions tetra-substituted phosphonium tetra-substituted borates in which phenoxy groups or the like are bonded to the phosphorus atom, namely P—O—C bonds are formed, and it discloses that the borates are effective as a catalyst enabling the encapsulating material to develop the storage stability at ordinary temperature; however, these tetra-substituted phosphonium tetra-substituted borates are easily hydrolyzed to produce phosphoric acid and there is a fear that the moisture resistance reliability may be remarkably deteriorated.